PTEN and glioblastoma: In this study, brain tumor cells from genetically engineered mice carrying activating mutations in the retinoblastoma protein (RB), receptor tyrosine kinase (RTK-RAS) and phosphatase and tensin homolog (PTEN) networks (the major signaling pathways altered in human GBM) were injected orthotopically into the brains of immune-competent, genetically identical background strains of mice in order to generate a GBM model.